Demand for flat panel displays is driven by a number of markets, including small and large consumer electronics applications, military applications, medical applications, handheld device applications, etc. Flat panel display sales have surged. The flat panel display market in 2004 was estimated by some to be a $62.2 billion market. Some expect this market to grow past $100 billion in a few years. In 2004, liquid crystal digital (LCD) displays accounted for about 78% of the $62.2B market.
Flat panel displays have obvious aesthetic and physical advantages compared to tube displays. Additionally, the picture quality in flat panel displays, e.g., plasma and liquid crystal digital displays has increased since the introduction of these technologies. Ongoing improvements in resolution and picture quality are being made, while the cost of flat panel displays continues to decrease.
In an LCD display, the positioning of a liquid crystal molecule column affects the polarization of light in a pixel. Each pixel in a display typically includes three sub-pixels. Each sub sub-pixel typically has one of three primary color filters (red, green or blue) and a black matrix. The display is backlit by a light source. Top and bottom polarizers are intended to block light when a sub pixel is “off”, and pass light to various degrees through the color filter when the electronics position the liquid crystal to polarize the backlight and permit it to pass through the color filter.
The LCD display model, while generally favored for low power, has some inherent drawbacks. The model relies upon two polarizer layers, which block a significant amount of light produced by the backlight. In the process of polarizing the light, the polarizer layer closest to the backlight blocks at least 50% of the light. Therefore, a large amount of electrical power is wasted to boost the light output from the backlight in order to produce a bright display. In portable electronics, e.g., laptop computers, personal digital assistants, and handsets, reducing the power consumption of the display remains important. The display, often an LCD display, remains the biggest single power draw in a typical portable electronic device.
Contrast, brightness and viewing angles are also issues in LCD displays. The relative lack of contrast and lack of brightness in LCD displays is attributable to the polarizer layers required by LCD displays. When a particular colored sub pixel is “on”, the polarizer layers permit the light to pass through the color filter and be emitted from the display, but the polarization filters portions of the available light and therefore substantially reduces the brightness of the light passing through. On the other hand, when a particular color sub pixel is “off”, the polarizer layers are intended to black the backlight, but effectively dim it to a great extent. An amount of light can be perceived as passing through the sub-pixel, reducing both the black levels and contrast ratios achievable with the LCD.
Recently, Philips has been researching and prototyping a different type of display, based upon the electrowetting principle. In the Philips prototype displays, electrowetting is used to move a colored oil layer away from a white background. The oil covers a hydrophobic layer, separating it from a water layer on top. Interaction of the oil, water, and the hydrophobic layer is controlled by an electric field. Application of an electric field across the layers causes water to contact the hydrophobic layer, displacing the oil, and the colored pixel is replaced by a white background.
Electrowetting is being researched by Philips and others, and various electrowetting display models have been published. U.S. Pat. No. 6,700,556, to Richley et al., entitled Display Sheet with Stacked Electrode Structure, discloses a display having alternating conducting and non-conducting sheets with fluid reservoirs. The reservoirs communicate with each other through apertures. Applying a field across selected apertures causes liquid to be pumped from one reservoir into the other, altering or producing a display based upon the position of the liquid. U.S. Pat. No. 5,956,005 to Sheridon, entitled Electrocapillary Display Sheet Which Utilizes an Applied Electric Field to Move a Liquid Inside the Display Sheet, also uses fluid movement between reservoirs for display effects.
U.S. Pat. No. 6,034,444 to Kawanami et. al., entitled Display Element and Display Device Having It, makes use of electronic control of liquid to focus or defocus light toward holes in a mask layer. In the '444 patent, first and second liquids are confined in a space between substrates. One of the liquids interacts with a patterned surface treatment layer. Under control of an electric field, the first liquid is controlled to have its contact angle with the patterned surface treatment layer changed. Light passing through the liquid is focused or not focused toward a corresponding small hole in the mask layer depending upon the contact angle of the first liquid.
U.S. Published application 2004/0196525 to Fujii et al, entitled Display Device discloses the use of fluid to either color or block light to produce a display. In the '525 application, a droplet of a coloring liquid is sealed in a cavity and controlled with an electric field to color or block light.